Copying .ppt from desktop to laptop - won't open

[rswc90]

When I copy my .ppt or .pptx from my iMac to my MBP, or vice-versa,
the program does not run. It freezes and will not open. I then have to
rename it -- mess with it for a very long time -- and when I am lucky,
I have a different version somewhere that I can work off of. What
causes this to happen?

Then, when I finally DO get it up and running, the videos won't
necessarily play. I have to delete that particular 1 minute video and
re-insert it. Then it will run fine. UGH!

Leopard 10.5.4 iMac intel -- NO windows installed! all updated
programs (including, unfortunately, firefox -- YUCK!)

Thanks in advance for any ideas.....